The invention relates to a method for producing a shaped sheet-metal part from a panel or a semifinished part made of a material consisting of steel with at least 60 wt. % Fe and a residual austenite content of at least 5%, in which the panel or the semifinished part is at least partially cooled to a temperature below −20° C. before the shaping and is shaped at a temperature below −20° C. in a forming tool. The invention furthermore relates to a device for carrying out the method and to an advantageous use of the sheet-metal parts produced.
In order to meet the increasing requirements to reduce weight, for example in motor vehicle manufacture, methods have been developed for producing shaped sheet-metal parts which, particularly under the term “hot forming”, undergo a pressure-hardening process in order to achieve maximum strengths, i. e. yield points and tensile strengths, in the pressure-hardened component. In this way, the wall thickness of the sheet-metal part, and therefore the weight, can be minimised. In this case, the panel or the semifinished part must usually be heated to a temperature above the AC1 transition temperature, so that the sheet-metal component essentially contains an austenitic structure, in order subsequently to be shaped at very high temperature and rapidly cooled. The effect achieved by this is that the austenitic structure is converted into martensite by the rapid cooling, so that very high tensile strengths and yield points can be provided. With manganese-boron steels, for example a manganese-boron steel of the type MBW1500, tensile strengths in the range of more than 1100 MPa can be provided by this method. The known hot-forming methods have furthermore been developed further so that the sheet-metal parts can also be locally provided with enormous yield points and tensile strengths, so that a load-compliant configuration of the sheet-metal parts can be achieved. The use of a “tailored blank”, which requires additional cost-intensive working steps in the form of a joining step, for example using a laser beam, or a separate component, can thereby be avoided. Disadvantages of hot forming are, on the one hand, the enormous energy outlay which is required for heating the panels or the semifinished parts to above the AC1 transition temperature, i. e. usually above 850° C. Furthermore, significant problems arise with surface coatings, which are required for example for corrosion protection. It is conventional to use hot-dip aluminised semifinished parts, or semifinished parts provided with an Al—Si coating, but these have no cathodic corrosion protection. Although surface coatings containing tin have cathodic corrosion protection, there is, however, the risk of melting the zinc on the surface during the heating. Uncoated semifinished parts are susceptible to scaling, if operation is not carried out in a protective gas.
The Japanese Patent Application JP 2000/178640 A, on the other hand, discloses a method in which the components are shaped at low temperature, and very high tensile strengths and yield points can thereby be achieved in the material by solidification. In the Japanese Patent Application, it is proposed to cool the components or at least partially using liquid oxygen, liquid nitrogen or dry ice, or in another way, and to shape them at temperatures of from −50° C. to −200° C. To this end, it is proposed to immerse the components in the corresponding refrigerants, in order to cool them very strongly. On the one hand, immersion of the sheet-metal shaped parts in liquid nitrogen or oxygen, or even dry ice, is not readily suitable for industrial-scale use. It furthermore entails risks for the operating personnel of corresponding plants, which lead to increased safety precautions.